(1) Field of the Invention
The present invention relates to a heterojunction bipolar transistor that has been widely used for a high power amplifier for transmission or the like and a method for fabricating the same.
(2) Description of Related Art
In recent years, with increase in functionality and communication capacity of cellular phones, higher performance has been demanded also for high-frequency analog elements used for cellular phones. Heterojunction bipolar transistors (hereinafter, referred to as “HBT”) out of the high-frequency analog elements have already been brought into actual use as high power amplifiers. In order to improve the performance of HBTs, the parasitic element effects need be reduced, i.e., parasitic resistances and parasitic capacitances need be reduced. The parasitic resistances are broadly grouped into an emitter resistance, a base resistance and a collector resistance. In order to reduce the contact resistance, HBTs have been suggested which use alloying reaction layers for ohmic electrodes.
The structure of a known HBT using alloying reaction layers for ohmic electrodes and a method for fabricating the same (see, for example, Japanese Unexamined Patent Publication No. 2001-308103) will be described hereinafter with reference to FIG. 7.
FIG. 7 is a schematic cross-sectional view showing the structure of the known HBT using alloying reaction layers for ohmic electrodes. As shown in FIG. 7, a subcollector layer 202 made of a heavily-doped n-type GaAs layer is formed on a semi-insulating substrate 201 of GaAs. A collector layer 203 made of a lightly-doped n-type GaAs layer, a base layer 204 made of a heavily-doped p-type GaAs layer and an emitter layer 205 made of an n-type AlGaAs layer are successively formed on a region of the subcollector layer 202 on which a collector is formed (hereinafter, referred to as “collector formation region”). In this relation, a predetermined part of the emitter layer 205 has a smaller thickness than the other part thereof and will be a base protection layer 205a. An emitter cap layer 206 made of a heavily-doped n-type GaAs layer and an emitter contact layer 207 made of a heavily-doped n-type InGaAs layer are successively formed on a region of the emitter layer 205 other than the part thereof that constitutes the base protection layer 205a. 
As shown in FIG. 7, an emitter electrode 211 with a Pt/Ti/Pt/Au structure (in which a Pt layer, a Ti layer, a Pt layer, and an Au layer are stacked in bottom-to-top order) is formed on the emitter contact layer 207. A base electrode 212 with a Pt/Ti/Pt/Au structure is formed on the base protection layer 205a made of an n-type AlGaAs layer. A collector electrode 213 with an AuGe/Ni/Au structure (in which an AuGe layer, a Ni layer and an Au layer are stacked in bottom-to-top order) is formed on a region of the subcollector layer 202 other than the collector formation region.
As shown in FIG. 7, a first Pt alloying reaction layer 214 is formed in a part of the emitter contact layer 207 located under the emitter electrode 211, and a second Pt alloying reaction layer 215 is formed in a part of the base protection layer 205a located under the base electrode 212. The first Pt alloying reaction layer 214 and the second Pt alloying reaction layer 215 are formed by heat treatment for reacting an electrode material (specifically, Pt constituting the lowest layer of each electrode) with a semiconductor material. The second Pt alloying reaction layer 215 passes through the base protection layer 205a and reaches the upper part of the base layer 204.
For the known HBT shown in FIG. 7, the provision of the base protection layer 205a prevents the recombination of carriers at the surface of the base layer 204. Thus, the current amplification factor is restrained from being reduced. On the other hand, since the base protection layer 205a covers the surface of the base layer 204, the base electrode 212 cannot be brought into direct contact with the base layer 204. To cope with this, the second Pt alloying reaction layer 215 is formed under the base electrode 212 by heat treatment to pass through the base protection layer 205a, thereby connecting the base electrode 212 through the second Pt alloying reaction layer 215 to the base layer 204. As a result, ohmic contact is obtained. On the other hand, the first Pt alloying reaction layer 214 located under the emitter electrode 211 is formed only inside the emitter contact layer 207.
As can be seen from the above, for the known HBT, the formation of the Pt alloying reaction layers 214 and 215 permits reduction in the widths of potential barriers at the junctions between the emitter contact layer 207 and the first Pt alloying reaction layer 214 and between the base layer 204 and the second Pt alloying reaction layer 215. This allows the tunnel effect of carriers to provide excellent ohmic characteristics. Therefore, the emitter contact resistance and the base contact resistance can be reduced, leading to reduced emitter resistance and base resistance.